Which number will replace the question mark (?) in the following series? 7,…
2020
Which number will replace the question mark (?) in the following series? 7, 17, 41, 85, ____, 257
- A.
147
- B.
145
- C.
150
- D.
155
Attempted by 14 students.
Show answer & explanation
Correct answer: D
Key idea: Look at the differences between consecutive terms.
17 - 7 = 10, 41 - 17 = 24, and 85 - 41 = 44.
The second differences are 14 and 20, increasing by 6 each time; the next second difference is 26, so the next gap is 44 + 26 = 70.
85 + 70 = 155. The next gap would be 102, giving 257, so the pattern is consistent.
Answer: Option 3, “155”, is correct.